A Pop Quiz for Einstein

May 24, 2000: Crystal balls rarely have anything to
do with science, but a special set of four soon may provide an
answer to one of the last, untested portions of Einstein's General
Theory of Relativity. Rather than peering into a crystal ball,
scientists will put four - each a manmade quartz gyroscope -
in orbit on a year-long Relativity Mission measuring how they
spin to see if the Earth's rotating mass distorts time and space.

Scientists
often compare the fabric of space to a rubber sheet, with the
Earth as a marble denting the surface, thus curving the paths
of passing objects. If Einstein is correct, then the rotation
of a planet or star also twists the sheet and distorts time,
just a little, in an effect called frame dragging that will slightly
repoint the gyros.

The gyros also will measure an effect called geodetic
precession, a miniscule compression of space caused by the
Earth's physical presence. Both the frame-dragging and geodetic
precession effects are so small that they require near-perfection
in the design and construction of the instrument, Gravity Probe
B (or GP-B).

Right: Under the glow of a green light, a scientist at
Stanford University checks for traces of dust on the quartz block
assembly that contains the four quartz gyroscopes at the heart
of the Relativity Mission. Credit: Stanford

"We've tried very hard to design an absolutely perfect
gyroscope," said Dr. Francis Everitt, the Principal Investigator
at Stanford University. Even in an age of exquisite measurements,
nothing is perfect. The GP-B gyros, though, are about as close
as humans can get. The gyros and their support system are so
precise that non-relativity effects will cause them to drift
by no more than 1/3 milli-arc-second during a year.

It's a number that Everitt knows well. He once took a loose
hair from his scalp and measured it in the machine shop at Stanford
University. At a distance of 32 km (20 miles), that hair would
appear to be a half milli-arc-second wide.

"That means the gyros can measure frame dragging to about
1 part in 150, and geodetic precession to about 1 part in 100,000,"
Everitt explained. But while he's confident about the accuracy
and precision of the answer, he won't predict the answer itself.